i have a 2003 mustang. the brake lights flash when i press the brake, however they immediately go off. any ideas. Thanks!

 

Pull the bulbs out and check that they are still good and look for corrosion.
Make sure the grounds aren't corroded.
Make sure the fuse is still in good shape.

If all that checks out, pull the bulb and have a buddy press the brake. While he does that use a volt meter to see if it is getting current. Follow the line back and check at each connection for current while the brake is pressed.

It may be anything from bad bulbs to a corroded connection or ground to a bad switch on the pedal. Good luck.